The present invention relates to a welding device for welding plastic strips, with a mounting frame and heating part to heat the strips.
When lining structures, for example tunnels, with plastic strips, the difficulty arises in welding because the plastic strips to be welded must provide a correct backing surface for the welding device or parts thereof; otherwise the weld will be defective. Structures to be lined generally do not have smooth surfaces which can serve as backing surfaces for the plastic strips during the welding process, so that an attempt has been made to remedy this shortcoming when lining such structures by bending the ends of the strips backward in the shape of the letter L and welding their undersides together. This may be successful in the case of thin plastic strips since the welded point can be shifted to one of the levels of the plastic strip and welded there again. This method is not practical for thicker plastic strips, for example those several millimeters thick.
Therefore, an object of the invention is to provide a welding device which is independent of the nature of the surface to be welded and still makes it possible to perform lap welding of the two ends or edges of the plastic strip in their planes, said welding being both qualitatively perfect and economically feasible.
A welding device of this kind is characterized according to an important aspect of the invention by a lower welding shoe which is provided directly beneath an upper welding shoe; said lower welding shoe forming a part of the welding device and being mechanically firmly mounted to the mounting frame or the upper welding shoe in order to form a unit which is supported exclusively on the material being welded during the welding process. With this arrangement, one can achieve a reliable and good weld even with a poor backing.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration, several embodiments in accordance with the present invention .